The invention relates to “active implantable medical devices” as defined by Directive 90/385/EEC of 20 Jun. 1990 of the Council of the European Communities. The invention may relate to implantable devices for continuous monitoring of the heart rhythm and delivery of electrical stimulation or resynchronization pulses, if necessary, to the heart. Embodiments of the invention may relate, more specifically, to pacemaker leads to be implanted in the cardiac coronary network to allow stimulation of a left, ventricular or atrial cavity.
A trend in recent developments in left ventricle pacing lead is the reduction of the diameter of the implantable part in the coronary network. The size of the lead body is indeed a factor directly related to the controlled guiding capacity of the lead in the venous coronary network, to be able to select specific stimulation sites located in certain collateral veins.
EP 2581107 A1 (Sorin CRM SAS) describes a lead composed in its active distal part by a microcable having a diameter of the order of 0.5 to 2 French (0.17 to 0.66 mm). This microcable includes an electrically conductive core cable formed by one or more strands of a plurality of composite strands. The microcable has a polymer insulation layer partially surrounding the core cable. The isolation layer is punctually exposed so as to expose the microcable in one or more points constituting a network of electrodes connected in series. The free end of the strand is also provided with a reported distal electrode.
The very small diameter of the microcable allows exploiting the entire length of the vein and cannulation of veins of very small diameter. These portions of the coronary network have generally not been exploited until now due to the excessive size of conventional coronary leads. It thus becomes possible to treat areas difficult to reach, and thereby make optimal use of all the veins present in the basal area. One benefit is a reduced risk of phrenic nerve stimulation; such risk generally increases when the lead is too distal. With such a microlead, it is even possible to cross anastomosis (passages present from the end of certain veins to another vein) with the possibility of advancing the microlead in a first vein (“go” vein) followed by an anastomosis into a second vein (“return” vein) going back thereof. This allows for stimulation of the left ventricle from two distinct and remote regions.
Moreover, the multiplication of stimulation points in a deep zone of the coronary network allows (unlike traditional leads) simultaneous stimulation of multiple zones of the epicardium in the region of stimulation, thereby improving the chances of myocardium optimal resynchronization. Finally, the structure of this microlead gives it great strength that improves its long-term biostability.
Another set of issues relates to the biological phenomenon of inflammation of tissue that is in mechanical contact with the lead. This contact exerts a pressure and is sometimes accompanied by small movements. These mechanical actions result in tissue inflammation, over the course of a few weeks. This is also a phenomenon encountered with all types of leads, whether they are placed in the coronary veins or in the cardiac chambers. In terms of device operation, this inflammation requires increased energy of delivered pulses due to the increase in the stimulation threshold (capture threshold). This also makes reassessment at regular intervals of the capture threshold necessary, so as to adapt the level of the energy delivered to the variations of this threshold according to the degree of inflammation. In summary, inflammation induces energy consumption of the device, and a risk of loss of capture.
The conventional technique to reduce this inflammation phenomenon is to incorporate to the leads molded silicone parts loaded of an anti-inflammatory agent. The anti-inflammatory agent is generally a steroid, e.g. a glucocorticoid such as dexamethasone sodium phosphate (DSP hereinafter). These molded parts are arranged in particular at the electrodes, to mitigate the effects of inflammation on the elevation of the capture threshold. The anti-inflammatory agent is gradually released by diffusion into and out of the silicon after implantation of the lead.
U.S. Pat. No. 5,496,360 A is an example of a lead designed according to this technique, with a hollow end electrode including an internal chamber. The internal chamber includes a polymer matrix impregnated with an agent such as the DSP. The chamber communicates with the outside through a narrow axial channel for the DSP to diffuse slowly around the electrode by effect of “osmotic pump” with the surrounding fluids. However, this technique is complex and expensive to implement because of the multiple process steps it involves: screening of the steroid to control the grain size, mixture preparation of silicone paste and steroid, molding and trimming, visual inspection of molded parts, and manual integration, lead by lead, of the silicone parts containing steroid. Further, this technique is in practice difficult to envisage for a microlead of a diameter less than 0.5 mm. The extremely fine size of such a microlead makes inadequate the implementation of reported silicone parts loaded with a steroid, or the machining of hollow parts as disclosed in the U.S. Pat. No. 5,496,360 A cited above.